Induction-coupled plasma synthesis of boron nitride nanotubes

ABSTRACT

Described herein are processes and apparatus for the large-scale synthesis of boron nitride nanotubes (BNNTs) by induction-coupled plasma (ICP). A boron-containing feedstock may be heated by ICP in the presence of nitrogen gas at an elevated pressure, to form vaporized boron. The vaporized boron may be cooled to form boron droplets, such as nanodroplets. Cooling may take place using a condenser, for example. BNNTs may then form downstream and can be harvested.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Nonprovisional ApplicationNo. 14/529,485 filed Oct. 31, 2014, which claims the benefit of U.S.Provisional Application No. 61/898,542, filed Nov. 1, 2013, the contentsof which are incorporated by reference in their its entirety.

STATEMENT REGARDING GOVERNMENT SUPPORT

None.

FIELD OF THE APPLICATION

This application relates to processes and apparatus for the large-scalesynthesis of boron nitride nanotubes (BNNTs).

INTRODUCTION

BNNTs are useful, inorganic, long-chain molecules. Wang et al., RecentAdvancements in Boron Nitride Nanotubes, Nanoscale, 2010, 2, 2028-2034,provide a recent summary of the structure, property, and uses of boronnitride nanotubes (BNNTs). Wang et al. also list several contemporarytechniques used to synthesize BNNTs. These contemporary techniquesinclude, for example: plasma-enhanced pulsed-laser deposition (PE-PLD),plasma-enhanced chemical vapor deposition (PE-CVD), pressurizedvapor/condenser method (PVC), arc-discharge, laser vaporization, BNsubstitution method from CNT templates, chemical vapor deposition (CVD)using borazine, induction heating boron oxide CVD (BOCVD),high-temperature ball milling, and combustion of Fe₄N/B powders.

These methods can be broadly divided into two categories: hightemperature synthesis and low temperature synthesis. An example of lowtemperature synthesis (e.g., synthesis at a temperature well below thevaporization temperature of boron) is BOCVD. A common high temperaturesynthesis approach (e.g., using temperatures above the vaporizationtemperature of boron) is the PVC method, which is also generallyreferred to as high temperature and pressure synthesis, or HTP.Typically, existing HTP methods produce long, flexible, single-to-fewwalled BNNTs (mostly two walls and less than about six walls, and lessthan about 10 nm diameter) with few defects and parallel concentricwalls. We have observed in previous work, HTP-grown BNNTs as long as 200microns. HTP synthesis is accomplished without the use of catalysts, orother intermediary species. With low temperature methods, BNNT formationrequires catalysts or additional precursors, for instance Li₂O, or MgO,in addition to boron and nitrogen feedstocks. Often, CVD grown tubeshave smaller aspect ratios, grow to much larger diameters (near 100 nmdiameter), and are less flexible than HTP-grown tubes. A severeshortcoming of current HTP implementations is low production rate,typically only 10's of milligrams per hour using kW-class lasers as theheat source.

As is evidenced by the current lack of any commercial source of BNNTs,all of the contemporary synthesis methods have severe shortcomings,including one or more of having low yield, short tubes, discontinuousproduction, and, most limiting, poor crystallinity (i.e., many defectsin molecular structure). What is needed is a commercially-viable BNNTsynthesis process that can produce long, flexible, molecules with fewcrystalline structure defects, and significant yields over contemporaryprocesses.

SUMMARY

Incorporating inductively-coupled plasma (ICP) into BNNT synthesisresults in a commercially viable process that can produce long,flexible, molecules with few crystalline structure defects, andsignificant yields over contemporary processes. ICP is described in U.S.Pat. No. 3,694,618 and U.S. Patent Publication 2012/0261390, forexample, the contents of which are incorporated by reference in theirentirety. Although ICP has been used in the synthesis of variousnanopowders and carbon nanotubes, BNNTs grow by a different chemicalmechanism than carbon nanotubes (CNTs). ICP methods for such synthesisare therefore inapplicable to BNNT synthesis. Moreover, ICP allows theBNNT synthesis process to achieve pressure and temperature conditionsthat drive the HTP synthesis method at much higher production rates andimproved end product quality and yield than would be expected. Forexample, in BNNT synthesis, a target critical temperature for HTP is thevaporization temperature of boron, which is above approximately 4000 Cat atmospheric pressure. A target critical pressure is generally between2 and 12 atmospheres. ICP may be used in BNNT synthesis to achieve thosetarget conditions, and produce commercially-viable and high quality(e.g., long, single-to-few walled, high-aspect ratio, highlycrystalline) products. Additionally, the ICP-based BNNT synthesismethods described herein may be performed in the absence of catalysts orintermediary species. For instance, only boron and nitrogen participatein the BNNT self-assembly process in some embodiments of the methods,such that the reactants consist essentially of nitrogen and theboron-containing feedstock. (Impurities in the boron-containingfeedstock are not considered reactants in this disclosure.) Catalystsand intermediary species, such as H₂, are not necessary in suchembodiments, thereby making the methods described herein moreeconomically viable.

The ICP-based BNNT synthesis methods described herein offer additionaladvantages. For example, ICPs offer several unique and importantadvantages as the energy source used to drive the HTP BNNT formationmethod. Specifically, ICPs permit continuous operation of the synthesisapparatus, provide the high temperatures across a broad spatial regionthat result in uniform nanotube precursors, and produce low flowvelocities/high residence times to enable complete vaporization of theboron feedstock, nucleation of boron droplets, and growth of BNNT. Also,ICPs have no consumable electrodes. Electrode erosion is a persistentproblem in transferred and non-transferred AC or DC electric arcs, as itcan introduce foreign species into the reaction zone that may inhibitthe formation of BNNTs and/or contaminate the nanotubes. ICPs have beenconstructed and tested at power levels up to 100 kW, which will enablehigh throughput synthesis of BNNTs. The improvements associated withusing ICP-based synthesis enable the manufacturing of high quality BNNTson the industrial scale of kilograms per day, as is currently achievedwith other ICP-synthesized materials, like micron-sized metal powders.

An object of this disclosure is to present new methods of BNNT synthesisto overcome the failings of contemporary synthesis methods, and generatehigh quality nanotubes in large quantities and in commercially-viableprocesses. Generally, high quality BNNTs have about two to six walls, adiameter of less than about 10 nm, and an aspect ratio greater thanabout 1000 and as much as about one million. The methods describedherein use ICP to drive an HTP method of BNNT synthesis at greatlyincreased production rates, including more than ten times the ratesachieved using contemporary methods.

Embodiments of a process for synthesizing boron nitride nanotubes(BNNTs) may include introducing nitrogen gas and a boron-containingfeedstock into a chamber, heating the nitrogen gas and boron-containingfeedstock with an induction-coupled plasma to form heated reactants,introducing asperities downstream of the heated reactants to triggerformation of BNNTs, and collecting the BNNTs. The heated reactants maycomprise boron nanodroplets. The BNNTs may be single-to-few walled, andmay be greater than 50 microns long. The asperities comprise, forexample, a condenser. The condenser may be, as examples, a copper rod, atungsten wire, a network of copper rods, a network of tungsten wires, agrid of copper rods, a grid of tungsten wires, or a combination. In someembodiments, the process proceeds without catalysts, i.e. with noreactive feedstock other than boron and nitrogen. In some embodiments,the reactants are maintained at an elevated pressure, such as anelevated pressure of at least 2 atmospheres to about 250 atmospheres,or, for example, an elevated pressure of at least 2 atmospheres to about12 atmospheres.

Embodiments of a process for synthesizing BNNTs may include dispersingreactants such as nitrogen gas and a boron-containing feedstock into achamber at an elevated pressure, heating, with an induction-coupledplasma, the reactants to a temperature greater than the vaporizationtemperature of the boron-containing feedstock, to form vaporized boron,cooling the vaporized boron to form liquid boron (such as boronnanodroplets), exposing the liquid boron and nitrogen gas to atemperature between boron's melting point and below boron's boilingpoint; and harvesting BNNTs. Boron-containing feedstock may include, asexamples, elemental boron, elemental boron powder, boron nitride, boronnitride powder, cubic boron nitride powder, and hexagonal boron nitridepowder. In some embodiments, cooling the vaporized boron is performedwith at least one of a condenser and a plurality of asperities. In someembodiments, cooling the vaporized boron may be performed by introducinga plurality of asperities in the form of a condenser.

An apparatus for ICP-based BNNT synthesis may include a chamberconfigured to maintain nitrogen gas and a boron-containing feedstock atan elevated pressure, a gas inlet configured to introduce nitrogen gasinto the chamber at an elevated pressure, a feedstock inlet configuredto disperse a boron-containing feedstock into the chamber, aninduction-coupled plasma head, a boron droplet nucleation zone, a growthzone, and a collection zone.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an embodiment of a process for ICP-based BNNTsynthesis.

FIG. 2 shows an embodiment of an apparatus for ICP-based BNNT synthesis.

DETAILED DESCRIPTION

In accordance with the principles described herein, high quality BNNTsmay be synthesized in large, commercially-viable quantities, using ICP.Some embodiments produce commercially-viable and high quality productswithout the use of catalysts or intermediary species. Embodiments of theprocesses disclosed herein employ an elevated pressure to drive the BNNTsynthesis. Some embodiments use a condenser or other source ofasperities to trigger BNNT formation. Some embodiments use naturalnucleation of boron droplets (i.e. not induced by forced local cooling).

ICP-based BNNT synthesis may be performed at elevated pressure, such asbetween 2 atmospheres and 250 atmospheres, and for example, between 2atmospheres and 12 atmospheres, and as a further example, at about 12atmospheres. As described in U.S. Pat. No. 8,206,674 B2 to Smith et al.,the contents of which are incorporated by reference in their entirety,BNNT synthesis may be performed at elevated pressures, including forexample pressures above 1 atmospheres and, for example, between 2atmospheres and 250 atmospheres. The methods Smith et al. describeemploy a laser thermal source to heat a target in a chamber at elevatedpressure, from which vapors condense to produce BNNTs without the use ofa catalyst. Super-atmospheric pressure is an important component forenhanced HTP methods, and is believed to greatly enhance thesupersaturation of boron nitride on the boron droplets which act asnucleation sites for the formation of BNNTs. Smith et al. describe 2 to250 atmospheres as a suitable range, and describe that 2 atmospheres to12 atmospheres produces excellent results. Similarly, U.S. Pat. No.8,753,578 B1 describes a laser-based apparatus and recommends 12atmospheres as a desirable pressure condition. U.S. Patent Publication2013/064,750 describes using an arc discharge to produce a plasma jetfor the production of BNNTs typically at 600-700 torr but possibly at“several to several hundred atmospheres.”

FIG. 1 is a flow chart of an embodiment of a process for ICP-based BNNTsynthesis. In step S101, a boron-containing feedstock is dispersed intoa chamber containing nitrogen gas held at a pressure suitable for HTPBNNT synthesis. The pressure may be an elevated pressure, as describedelsewhere herein. An advantage of using ICP is that the feedstock may becontinuously fed into the chamber. The feedstock may be fed to thechamber using a mechanical disperser such as a powder disperser, forexample, among other known methods for feeding a feedstock to a chamber.For instance, liquid spray injection (atomization) of theboron-containing feedstock may be used, particularly when the solventproduces no chemical interference in the dispersion and synthesisprocesses.

The boron-containing feedstock introduced at S101 should contain boron,and preferably the boron is in an easily dispersed form. For instance,elemental boron can be dispersed, but many forms are gritty and sufferfrom inconsistent or less-than-desirable dispersion. Boron nitride (BN)powders may be preferred for many methods. In some methods, BN releasesnitrogen during vaporization, and may require the ICP to have additionalheating capacity. Cubic BN powders may be suitable, although thediamond-like structure can negatively impact the material flow throughthe apparatus. Both B and BN powders are available in dozens ofindustrial grades. Typically smaller particles are harder to disperse,but they vaporize more easily. The best feedstock is one that vaporizescompletely with available power and residence time, but costs the leastand has the highest throughput. With laser-driven HTP, elemental boronpowder and hexagonal boron nitride powder feedstocks have been found toproduce good results, generating high quality BNNTs, including BNNTsover 50 microns in length. High resolution transmission electronmicroscopy (TEM) has been used to confirm the quality of BNNTs producedusing elemental boron powder and hexagonal boron nitride powder asboron-containing feedstock.

In step S102, the introduced boron feedstock is heated by ICP to produceboron gas (vapor). Vaporization requires that the dispersed borontemperature be held above its vaporization temperature for a timesufficient to convert it to gas. The duration is dependent on at leastthe size of the feedstock particles and the local temperature, buttypically may be on the order of about 1 millisecond to 100s ofmilliseconds. Complete conversion of feedstock to vapor is readilydetermined by observing under what conditions no residual powderedfeedstock is present downstream of the ICP hot zone.

The operating pressure within the chamber (which includes the ICPvolume) should be sufficient to drive the HTP method. Generally, anelevated pressure of between 2 and 250 atmospheres will drive thesynthesis and achieve high quality BNNTs at strong yields. In someembodiments, the operating pressure is between 2 atmospheres and 12atmospheres. For example, pressures from about 4 atmospheres to 20atmospheres have repeatedly produced good results using HTP methods.Additional gases may be employed to improve the performance of the ICP.For instance He or Ar may be added to the ICP to vary the temperatureprofile within the ICP or to facilitate easier ignition of the plasma.Noble gasses such as He and Ar are not chemically reactive, and shouldnot participate chemically, although as buffer gasses they can alter thekinetics. For plasma ignition scenarios, added gasses can be turned offonce the plasma is established. Thus, as described herein, the use of anoble gas for plasma ignition does not constitute the use of a catalystor intermediary species.

In step S103, the boron vapor, now dispersed within the nitrogen carriergas, is cooled to create small boron droplets. It can be extremelydifficult to measure the size of the droplets in situ (i.e., in theplasma) but examination of post-run HTP samples suggest that activedroplets are in the range of about 2 nm to about 500 nm, i.e.,nanodroplets. In some embodiments, the boron vapor cooling can occurwith the natural fall off in temperature as a function of distance fromthe ICP head. In some embodiments, the boron vapor cooling may belocally induced by a condenser, such as described in U.S. Pat. No.8,753,578 at column 2, line 3, through column 3, line 36, which areincorporated by reference. The condenser can take many forms, such as,for example, a cooled copper rod or tungsten wire or networks or gridsthereof. Primary considerations are that the condenser is capable ofsurviving the ambient temperature of the boron droplet nucleation zoneand that the flowstream can pass readily over/through it. Someembodiments may induce boron vapor cooling using asperities, such asdescribed in U.S. Pat. No. 8,206,674 at, for example, column 2, line 53through column 6, line 14, which are incorporated by reference. Anasperity may also comprise bump, protrusion, or indentation on asurface, such as a metallic surface of a condenser. Generally, anasperity may cause local cooling of the boron vapor flow and subsequentformation of boron droplets or nanodroplets. Some embodiments feature aplurality of asperities that when combined provide a surface thatinduces the continuous formation of boron droplets from vapor phase.Some embodiments may feature a combination of a condenser and aplurality of asperities. In some embodiments, the condenser includesasperities. The condenser and/or asperities can be used to control wherethe droplets form, and may enhance the purity of the BNNT product byextending the time in the growth zone. Some embodiments use naturalnucleation of boron droplets (i.e. no forced local cooling). We term thelocation where the boron droplets form the ‘nucleation zone.’

In step S104, the condensed boron droplets are held at a temperatureabove the melting point of boron and below the boiling point of boronfor some time in the elevated-pressure nitrogen environment. In someembodiments, step S104 occurs in a ‘growth zone,’ in which BNNTs form.The streamwise length of the growth zone can be controlled by acombination of the operating conditions of the ICP and thermalinsulation on the walls of the chamber. It is believed that during thetime condensed boron droplets are held at a temperature above meltingpoint, the droplets extrude BNNTs through a process of localsupersaturation of BN on the surface of, or within the volume of, theliquid boron droplets. This self assembly mechanism is rapid and mayproceed without additional chemical components (catalysts such as Li₂Oor MgO or intermediaries such as H₂), particularly at elevated pressuresas described above. Thus, in some embodiments, the chemical reactantsconsist essentially of nitrogen and boron (as mentioned above,impurities in the boron-containing feedstock, if any, are not consideredcatalysts or reactants in this disclosure).

In step S105 the nanotubes are harvested, i.e., separated from thenitrogen carrier gas. For the bulk collection of BNNT material, screensor wire mesh filters (for instance stainless steel wire with 5 mm to 1cm spacing) consistently produce good results. For large-scaleembodiments, centrifugal separators may be employed in conjunction withscreens. Dry spun structural fibers may also be produced by feeding thecotton like BNNT raw material into dry spinners and weavers. U.S. Pat.No. 7,993,620, incorporated by reference in its entirety, describes asimilar process for carbon nanotube fiber.

FIG. 2 shows an embodiment of an apparatus 1 for ICP-based BNNTsynthesis as described herein. Apparatus 1 may comprise an enclosedchamber, and may have a generally tubular or other convenient crosssection. The apparatus 1 may have an ICP region 2, a boron dropletnucleation zone 3, a BNNT growth zone 4, and a collection/harvestingzone 5. The nitrogen carrier gas is introduced to the chamber throughgas inlet 6, and the gases exhaust the chamber at exhaust 7. OptionalICP augmentation gases, as described above, may be introduced to thechamber at gas inlet 6. Embodiments of the apparatus 1 may feature aplurality of gas inlets 6. The chamber pressure can be set withconventional gas regulators on the upstream side (not shown in FIG. 1),and the flow rates regulated by conventional variable valves at theexhaust. As described above, operating at an elevated pressure ofbetween about 2 atmospheres to 250 atmospheres, and as an examplebetween about 2 atmospheres to 12 atmospheres, and as a further exampleat about 12 atmospheres, drives the ICP-based BNNT synthesis process andgenerates good yields of high quality BNNTs.

The boron-containing feedstock may be introduced to the chamber througha feedstock inlet 8. The feedstock inlet 8 may be, for example, amechanical disperser or an atomizer, among other devices for feeding theboron-containing feedstock to the apparatus 1. The conventional ICP headincludes induction coils 9, positioned around the body of the head todrive the plasma within. The ICP creates a hot region 10 with sufficienttemperature to vaporize the chosen feedstock. Embodiments of theapparatus 1 may include condenser 11 that can be positioned in the borondroplet nucleation zone 3, just upstream of the growth zone 4. Thecondenser can be used to induce nucleation of boron droplets at the mostfavorable streamwise location within the chamber. A favorable locationis one that allows the droplets to be held at a desirable temperaturefor the extrusion of BNNTs for a time sufficient to completely consumethe feedstock. Condenser 11 may comprise, for example, a cooled copperrod or tungsten wire or networks or grids thereof. If natural nucleationof boron droplets is used (i.e. not induced by forced local cooling),the condenser can be omitted.

Cotton-like BNNTs 12 form in the growth zone 5. The growth zone persistsover the region where the temperature is sufficiently high that theboron droplets continue to react vigorously with the ambient nitrogen.Some theories suggest that once the boron droplets solidify, BNNT growthis extinguished, or greatly diminished. BNNTs can be collected from thecondenser 11 if present, or in the collection zone 13. Harvesting in thecollection zone 13 may be accomplished using a number of techniques,such as those described above.

Collection of BNNTs can easily be achieved in batch mode, wheresufficient volume is left for the deposition of BNNTs in the collectorzone. The BNNTs may be harvested from the volume between operationalruns. For a continuous BNNT fiber, the fiber may be spun under pressure(region 13 in FIG. 2) and the fiber passed out of the chamber through arotating seal (located at position 7 in FIG. 2), or it could becollected on a roll within the pressurized chamber volume.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the approach. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. it will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the claims of the application rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A process for synthesizing boron nitride nanotubes (BNNTs), the process comprising: a. introducing a feed consisting of gas including nitrogen and a boron-containing feedstock into a chamber; b. heating the nitrogen gas and boron-containing feedstock with an induction-coupled plasma to form heated reactants; c. triggering formation of BNNTs; and d. collecting the BNNTs.
 2. The process of claim 1, wherein the formation of BNNTs is triggered by at least one of introducing asperities into the chamber, natural nucleation, and cooling with a condenser.
 3. The process of claim 1, wherein the formation of BNNTs is triggered by at least one of a copper rod, a tungsten wire, a network of copper rods, a network of tungsten wires, a grid of copper rods, and a grid of tungsten wires.
 4. The process of claim 1, wherein the process proceeds with no reactive feedstock other than boron and nitrogen.
 5. The process of claim 1, wherein the collecting is performed using at least one of a solid surface condenser, collection surfaces, and filters downstream of the growth zone.
 6. The process of claim 1, wherein introducing nitrogen gas and a boron-containing feedstock into a chamber is a continuous process.
 7. The process of claim 1, wherein collecting BNNTs is a continuous process.
 8. The process of claim 1, wherein the chamber is at an elevated pressure of at least 2 atmospheres to about 250 atmospheres.
 9. The process of claim 8, wherein the chamber is at an elevated pressure of at least 2 atmospheres to about 12 atmospheres.
 10. The process of claim 1, wherein the heated reactants comprise boron nanodroplets.
 11. The process of claim 1, wherein the boron-containing feedstock comprises at least one of elemental boron, elemental boron powder, boron nitride, boron nitride powder, cubic boron nitride powder, and hexagonal boron nitride powder.
 12. The process of claim 1, wherein the feed is introduced at a feed inlet, and the heated reactants are formed downstream of the feed inlet.
 13. A process for synthesizing boron nitride nanotubes (BNNTs), the process comprising: a. continuously introducing reactants including nitrogen and a boron-containing feedstock into a chamber to form a reactant feed; b. passing the reactant feed through an induction-coupled plasma to heat the reactant feed at least the vaporization temperature of the boron-containing feedstock to form a heated reactant feed; c. triggering formation of BNNTs from the heated reactant feed; and d. collecting the BNNTs.
 14. The process of claim 13, wherein triggering the formation of BNNTs is by at least one of introducing asperities into the chamber, natural nucleation, and cooling with a condenser.
 15. The process of claim 13, wherein the formation of BNNTs is triggered by at least one of a copper rod, a tungsten wire, a network of copper rods, a network of tungsten wires, a grid of copper rods, and a grid of tungsten wires.
 16. The process of claim 13, wherein the process proceeds with no reactive feedstock other than boron and nitrogen.
 17. The process of claim 13, wherein the collecting is performed using at least one of a solid surface condenser, collection surfaces, and filters downstream of the growth zone.
 18. The process of claim 1, wherein collecting BNNTs is a continuous process.
 19. The process of claim 1, wherein the chamber is at an elevated pressure of at least 2 atmospheres to about 250 atmospheres.
 20. The process of claim 1, wherein the feed is introduced at a feed inlet, and the heated reactants are formed downstream of the feed inlet. 